When you compress the data files, you are compressing the data files in your folders.
Basically, what happens with a database is this. Say you add a name to your database.
"John"
Now you add three more names.
"John"
"Sara"
"Susan"
"Mike"
Now, let's say you delete Sara and Susan, but add five more names.
"John"
""
""
"Mike"
"Bill"
"Allan"
"Martha"
"Diane"
"Greg"
Now you delete Allan and Martha.
"John"
""
""
"Mike"
"Bill"
""
""
"Diane"
"Greg"
What I'm trying to illustrate is that deleting things from your database doesn't actually free up the space those items were taking. The size of your database file grows as you add records, but it never shrinks when you delete records. The illustration above isn't entirely accurate, because when you add records, it will fill in the blank space before growing the file, but this is just for a quick illustration, not an in-depth tech analysis.
So now you have this as a database file:
"John"
""
""
"Mike"
"Bill"
""
""
"Diane"
"Greg"
And you run the compression tool. The compression tool frees up all the empty space from the database file. So you end up with this:
"John"
"Mike"
"Bill"
"Diane"
"Greg"
Depending on the frequency and the dynamics of when and how you add and delete data, combined with how long it's been since your last compression, the difference after compressing your data could negligible or it could be dramatic. It could shrink your database by several gigabytes, or only by a few bytes. It all depends.